The present invention relates to rotary drilling, and particularly to flow optimization of jet bits during rotary drilling.
Background: Rotary Drilling
Oil wells and gas wells are drilled by a process of rotary drilling, using a drill rig such as is shown in FIG. 1. In conventional vertical drilling, a drill bit 10 is mounted on the end of a drill string 12 (drill pipe plus drill collars), which may be several miles long, while at the surface a rotary drive (not shown) turns the drill string, including the bit at the bottom of the hole.
Two main types of drill bits are in use, one being the roller cone bit, an example of which is seen in FIG. 4. In this bit a set of cones 16 (two are visible) having teeth or cutting inserts 18 are arranged on rugged bearings such that when rotated about their separate axes, they will effectively cut through various rock formations. The second type of drill bit is a drag bit, having no moving parts, seen in FIG. 3.
During drilling operations, drilling fluid, commonly referred to as "mud", is pumped down through the drill string and out holes 28 in the drill bit 10. The flow of the mud is one of the most important factors in the operation of the drill bit, serving at least three purposes: to remove the cuttings which are sheared from rock formations by the drill bit, to cool the drill bit and teeth, and to wash away accumulations of soft material which can clog the bit. (The flow of mud also serves many other purposes, e.g. to lubricate the bearings of some rotary bit designs.)
Originally, mud was directed at the rotating roller cones, with the purpose of cleaning the cones. With the use of jet bits, in which velocities of a hundred feet per second to several hundred feet per second are common, the mud is currently directed toward the hole bottom. The turbulence created by the stream of mud will clean the bit, as well as carry away rock chips.
Background: Nozzles
Within the aperture where mud leaves the bit, removable flow-restrictors, called nozzles, determine the size of the opening, and therefore the final velocity of the mud stream. An example can be seen in FIG. 2. In this figure, a nozzle 20 has been inserted into the aperture 14, where it fits snugly. It can be held in place by any one of several means, such as a snap ring 22 (often shrouded to protect the ring from erosion from the mud), screw threads, or a nail lock (where a flexible "nail" is inserted from the edge of the bit to fit into a groove on the outside of the nozzle and inside of the aperture, locking the nozzle in place). At the inside end of the nozzle, its inside diameter is approximately that of the opening above it, while at its outside end, the diameter can be whatever is desired to give the final flow characteristics. To adjust the flow, the nozzle can be replaced with another nozzle which has a different internal diameter at the outside end.
The final inside diameter at the outside end of the nozzle is measured in increments of 1/32 of an inch, and for a single bit having a given aperture, it is not uncommon to stock 20 different sizes of nozzles. Additionally, the size of nozzle needed can not be determined in advance, only estimated, as many factors affect the choice. Thus, when a bit is shipped to the drill rig site, it is common to send perhaps four nozzles for each aperture, in appropriate sizes. The correct nozzles will be installed at the drilling site, while those which are not used are generally lost or discarded. The combination of high inventories and high waste of nozzles increases costs and wastes time, not only in the field, where the nozzles must be installed, but in the warehouse, where they must be tracked.
Variable Orifice Nozzle
The present application teaches a jet-bit nozzle which has an adjustable orifice, allowing the same nozzle to deliver the mud at variable pressures. This is accomplished by the use of two thick plates, each having a shaped aperture therein. The degree to which the two apertures are overlapped determines the size of the orifice. The movement of at least one of the plates, and thus the size of the orifice, can be adjusted at the drill site, to give a desired pressure drop across the nozzle.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:
the nozzle can be factory installed, assuring reliable installation and quality control; PA0 inventory can be reduced and wastage eliminated.